In practice, a bumper crossmember is usually connected to a vehicle structure at its two longitudinal ends via energy-absorbing elements, for example via so-called crash-boxes. The bumper crossmember directs impact energy into the energy-absorbing elements, in order to convert kinetic energy into deformation energy and to thereby sustain the vehicle structure in the event of a collision, in particular the left and right longitudinal beams of the vehicle chassis during severe accidents.
In the event of a head-on collision, in particular with reference to barriers that are arranged centrally in relation to the vehicle, the maximum bending moment in a bumper crossmember usually occurs in the middle of the crossmember, i.e. in the longitudinal direction centrally between the crash-boxes.
In the current state of the technology, the stability of the crossmember is increased in this central area by means of the following measures. On the one hand, the wall thickness of extruded crossmembers can be increased towards the center. An alternative way is for example, that the crossmember is compressed or reshaped in the direction of its depth of its longitudinal side end sections, i.e. in the areas that connect to the crash-boxes. Alternatively, such a variation of the thickness in the direction of the depth of the crossmember can also be achieved by means of a so-called hydroforming process, so that e.g. the central area of the crossmember can be pressed apart in the direction of its depth and therefore the thickness of the central area is increased compared to the longitudinal end sections of the crossmember.
An example from the prior art is shown in FIG. 7b, which shows a connecting area to a longitudinal end of a crossmember 100 as viewed in a cross-section that is taken in a plane perpendicular to the longitudinal direction of the crossmember. The cross-sectional view shown in FIG. 7b corresponds approximately to the location that is indicated by the line A-A in FIG. 1 for a crossmember according to an embodiment of the invention.
Referring still to FIG. 7b, the crossmember 100 is composed of an extruded two-chamber hollow profile with an upper chamber 101 and a lower chamber 102. On their respective front sides the two chambers 101, 102 are limited by baffle plate 103. Fixing holes 104 are provided within the connecting areas, into each of which a screw 106 is inserted and fixed via a nut 105, which couples a front end of a crash-box 107 to the crossmember 100.
The depicted end portion is compressed in a direction of the depth of crossmember 100, so that it features a reduced thickness D compared to the central portion (not shown) of the crossmember 100. The upper-, central- and lower walls (108, 110 and 112, respectively), which limit the chambers 101 and 102 between the baffle plate 103 and a rear wall 114 of the crossmember 100, are bent by means of this compressing in the direction of its depth. The results are problems that relate in particular to the mounting situation.
Overall, the construction of all crossmembers that are known from the prior art with a higher stability in the area between the connecting areas is complex and their weight is relatively high, which can have a negative effect on vehicle fuel efficiency.